Once Upon a Tail
by Supervillegirl
Summary: Previously titled "Baker Street Boys." The first in my Tail series. John Watson has managed the impossible: keeping a life-changing secret from world's only consulting detective, Sherlock Holmes. But now, the secret's out.


Baker Street Boys

 **I've been kinda working on this on and off at the same time as "Waiting for Time."**

* * *

Dr. John Watson slowly came to and attempted to rub a hand over his face to clear the cobwebs, attempted being the keyword. His hand didn't accomplish anything but a loud rattle as the chain around his wrist clanged against the wall. His head jerked up to look at his hand, which was shackled to the wall. He looked over to see his other hand similarly restrained. He looked across from him to see his friend Sherlock Holmes also chained to the opposite wall, but with a bored look on his face.

John let out a frustrated sigh, letting his head fall back against the wall. "'We'll be in and out,' you said. 'They'll never know we were here,' you said."

"I know," muttered Sherlock in an annoyed tone.

John brought his head away from the wall to glare at Sherlock. "This is a terrorist organization, Sherlock. You know what this means?"

"Yes, torture, dull," said Sherlock, his own head leaning back against the wall.

John closed his eyes, clenching his jaw. "I was thinking more along the lines of 'we're not getting out of here alive,' but, sure, torture is the one to really worry about."

"Oh, not to worry, I'll think of something," said Sherlock.

John glanced over at him. "Got anything yet?"

Sherlock hesitated uncomfortably before admitting in a low voice, "Not yet."

John sighed and dropped his head. _Great. Sherlock got us into this mess, and now, I have to get us out. This will go over well._

* * *

Sherlock looked over at John when he gave his sigh and dropped his head. He was truly sorry for not having seen this coming and getting them into this situation. But surely, they'll think of something. An opportunity will present itself. It always does.

John lifted his head and focused on the shackles on his wrist, brows furrowed in concentration. He then looked over at the door, his eyes darting to different points on it.

Sherlock narrowed his eyes at his friend. He knew a plan formulating when he saw it. But just what John planned to do in a locked, fortified room chained to a wall, he had no idea.

John then looked up at the security camera in the corner of the ceiling by the door. He narrowed his eyes and clenched his jaw momentarily, coming to a decision. He glanced at Sherlock for a second and then looked back up at the camera. His hand turned so that his palm was facing the camera, and then his fingers slowly began to curl in towards his palm. As his hand closed into a fist, Sherlock frowned and looked up at the camera. It had started smoking. Another few seconds, and the lens shattered as it let off a cracking sound and sparks. The red light on the face of it instantly blinked off.

Sherlock looked over at John, who unclenched his fist. "John…"

"I'm sorry, Sherlock," John told him in a rush. "But we don't have time for me to explain."

He turned his right hand towards his left one and turned his head, pressing it as hard as he could against the wall. He straightened his right hand out flat, the palm facing his other hand. Instantly, frost crept over the shackle on his left wrist. John then balled his left fist and gave a good yank. The shackle shattered as though made of ice. He then raised his left hand to do the same to his right shackle. Once he was free, he scrambled over to the other side of the cell, where Sherlock was staring at him in shock.

"Don't move," John told him, which was quite unnecessary; Sherlock could not think to move.

John quickly dispatched Sherlock's chains as he had his own. "Come on."

Sherlock could only stare, trying to wrap his logical, scientific mind around the fact that his friend of two months had superpowers. _Ridiculous word!_

John placed his hands on Sherlock's shoulders to get him to focus. "I know your mind is reeling, and I will tell you everything later, but they will be here any second. I need you to trust me."

Now, that was something Sherlock knew he could do. He got to his feet and let John usher him over to the corner by the door, where they would be hidden when it opened.

"Keep quiet," John told him as they heard hurried footsteps just outside their door. He then backed up into the room.

"What about you?" hissed Sherlock as the sounds of a key being put into the lock echoed in the room.

Stopping at the wall opposite the door, John gave a little smirk and then waved his hand in front of his face. He then vanished from sight. Sherlock's jaw dropped as the door was flung open in front of him. Unable to see anything behind the door, he could only listen as there was an explosion of angry shouts from what sounded like two men. Just as the footsteps approached the door, there were two yells and then a commotion; it sounded as though they had been thrown against the wall. Two seconds later, the door swung away from the wall, and John was standing there.

"Come on," he said, motioning Sherlock forward.

Sherlock glanced at the two guards lying unconscious on the floor and followed as John stepped up to the doorway and peered around it, checking the hall. He jerked his head, and they moved into the hall, moving quickly but quietly.

"Can't you just make us both invisible?" Sherlock whispered.

"It's exhausting for too long, especially with another person," John explained. "I need my strength if we have to fight our way out."

They reached the end of the corridor and turned the corner, heading up some stairs to find a locked door there. John raised his hand and aimed it at the doorknob, his fingers stretched out. There was the sound of the tumblers clicking before the knob turned and the door popped open slightly. John immediately darted through it.

 _Fire, ice, invisibility and telekinesis_ , thought Sherlock. _What next?_

Sherlock stopped next to John at the corner. John peeked around it and moved his head back, shaking it. Sherlock could hear many voices in the room beyond; it was too much for John to take on at once. John looked over at a stack of boxes across the way, and he held up a hand to tell Sherlock to wait. John waved his hand in front of himself and became invisible, and Sherlock felt him leave his side. After a few moments, John reappeared hiding behind the boxes. He raised his hand towards the ceiling on the other side of the room, his arm outstretched. He kept moving his hand back and forth in a circle, and the next moment, there were shouts.

Keeping his arm moving, John motioned to him, and Sherlock came out from behind the wall. He stepped forward and looked over to see about ten or twelve men all staring up at a storm cloud that had appeared in the room. Sherlock slowed for a moment in surprise before hurrying over to John.

"I only have one hand, so keep hold of my arm," John hissed at him.

Sherlock latched onto John's arm as John used his free hand in front of himself. John turned invisible next to him, and Sherlock looked down to see that he had as well.

"Go," said John, rising underneath Sherlock's hand.

Sherlock stood and moved with John, and they hurried around the terrorists towards the front door. They stopped in front of it and turned to face them.

"When I say go, go," John told him.

The storm cloud, which had begun to rain at some point, grew darker, turning almost black.

"Now!" hissed John urgently.

As Sherlock grabbed hold of the door handle and turned it, a bolt of lightning streaked from the cloud and thunder crashed throughout the room, thoroughly disguising the sound of the latch disengaging and the creak of the hinges. As Sherlock flung himself through the door and thus out of contact with John, his body blinked back into existence, and he threw himself behind the door, should any of the terrorists look. Another few seconds later, John appeared next to him, lowering his hand.

"Run," said John.

They had gotten ten feet from the door when angry shouts emitted from the building. They quickened their pace as they passed over the creek next to the building and heard footsteps giving chase. John turned and stopped, raising his hands in front of himself. The creek suddenly burst from the ground, creating a wall of water fifty feet tall. He then thrust one hand towards it, flat and palm facing out. The water froze instantly into a wall of ice.

They could just see the men on the other side, banging away at the ice and firing rounds into it, but it was too thick to penetrate. Smirking, Sherlock turned and continued running, John on his heels. They ran through the forest until they reached a road, where a black car was pulling up to a stop.

The driver got out so they could see him. "Mr. Holmes. Dr. Watson."

"At least Mycroft was keeping an eye on us," muttered John as they got into the car.

* * *

Sherlock and John sat in the private plane, on their way back to England. So far, Sherlock had kept his silence since they had not been alone yet. But now, with only the pilot in the cockpit and Mycroft's assistant sent to the back room by Sherlock, it was bound to come any second.

"So…" said Sherlock, eyeing John in the seat across the small aisle.

"Yeah…" said John, sighing before turning the seat towards his friend. "Go ahead."

Sherlock turned his seat also. "How did you do all that? Training? An experimental drug? Born this way?"

John blinked for a moment, frowning.

"What?" asked Sherlock.

John gave his head a shake. "Erm, I, er…expected something like, 'Why didn't you tell me?'"

Sherlock rolled his eyes. "Waste of a question. I already know."

"You do?" asked John.

"It's obvious," said Sherlock. "If anyone—namely the media or government—got word of it, your life as you know it would disappear, and you'd be labeled a freak. Trust me, I know what that's like."

John was brought back to all the times he had heard Sergeant Sally Donovan address or refer to him as "Freak." He nodded. "Right. Two's company."

Sherlock gave a slight smile.

"None of the above," John answered. "It was purely an accident." He gave another sigh and leaned forward, propping his elbows on his legs and preparing for a long story.

"When I was fourteen, my family went on a vacation in Ireland. We were camping in a patch of woods not far from the coast. One day, I went out to explore, and I got turned around. I spent so long trying to find my way back that the sun set, so I decided to find shelter until the morning. It wasn't long before I found a cave. At the back of the cave, I saw a concealed tunnel that led further in, and I thought, 'well, I'm already lost.'"

Sherlock smirked.

"Eventually, I came upon a pool of water inside a cavern with a hole in the roof of it right over the water," John went on. "It was pretty roomy, and there was some wood along the sides, so I decided to build a fire and stay there until daylight. Over the next few hours, it got pretty cold—even with a fire—and I noticed steam rising from the pool. Turns out, it was a sort of hot spring, so I jumped in to keep warm."

John hesitated a moment. _Here we go._ "Halfway through the night, light suddenly poured down into the room. It was a full moon that night, and it was starting to move overhead, and it shone down through the hole in the cave roof. Everything was fine until the moonlight touched the pool. The water started churning like it was a hot tub or something. And these…" he frowned, trying to explain it, "little beads of light—almost like sparks—started rising from the water. It went on for a while, but I never thought to get out." He shook his head, spreading his hands. "I don't know how to explain it, but it felt…right, you know."

"When the moon finally moved enough that the light left the pool, the water settled, as though nothing had happened. In the morning, I went back to the forest, found my way back to camp, and that was it."

"That was it?" asked Sherlock. "That doesn't explain things at all."

John closed his eyes in irritation.

"Oh," said Sherlock. "Sorry."

John looked back up at him. "It wasn't until Harry and I went to the beach for a swim that anything happened. I was wading into the water when, all of a sudden, I collapsed. It was like my legs just gave out. Harry saw me fall and hurried over, asking if I was all right, and then she got real quiet. I turned over in the water and looked up at her. She was staring at my legs in what looked like shock. I looked down at my legs, but…they weren't there." He hesitated, avoiding Sherlock's eyes. "What that moon pool had done to me…" he looked back up at Sherlock, "was turn me into a merman."

Sherlock's eyes lit up in interest. "Of course. Power over water, creating a storm…"

John frowned. "You're not laughing."

Now, it was Sherlock's turn to frown. "Why would I laugh? Was this supposed to be humorous?"

"No, it really wasn't, but…" John began, trying yet again to explain ordinary human behavior to a genius. "Usually, when someone finds out their friend is basically a mermaid, they tear the mickey out of 'em."

"Oh," said Sherlock, looking down at the floor and thinking. He then looked up again. "Would you like me to make fun of you?"

John smiled, shaking his head with a huffed laugh. "No, you do that enough already."

Sherlock's frown deepened. "I do? When?"

John waved off the topic of conversation. "What were your other questions?"

Sherlock didn't answer right away, apparently searching his mind palace for an instance he had ever made fun of John. After a moment, he shook his head with a slight shrug and came back to the present. "How does it work? You touch water and transform? Are you able to control it?"

"No, no, no control over it at all," John told him. "If any kind of water-based liquid touches my skin, I've got roughly ten seconds to either get every bit of it off my skin or get somewhere private."

Sherlock's frown had reappeared. "But I've seen you in the rain before. You've had water on you."

John gave a smile. "Little trick I've learned over the years. I use my ability to manipulate water to create a sort of skin-tight forcefield so it looks like the water is on my skin, but it never actually touches it."

"And that doesn't exhaust you like invisibility does?" asked Sherlock.

"Only if I stand out in the rain for, like, an hour, which never happens," John replied. "Manipulating water is much easier than manipulating light rays."

Sherlock nodded. "How do you shower?"

"I don't," said John. "I take a bath and then run the shower to make you think otherwise."

Sherlock frowned and opened his mouth to speak, but John knew the questions that were coming and decided to head them off.

"I use my powers to fill and drain the tub so there isn't the sound of splashing water," he explained.

Sherlock stared at him, surprised at the measures he had taken to keep this secret.

John shrugged. "Hey, look who I have for a flatmate. I had to think these things through."

"Hmm," muttered Sherlock. "I'm impressed."

John's brows rose. "By my bathing technique?"

"By the fact that you kept this a secret from me," said Sherlock. "Not an easy feat."

"No, it wasn't," muttered John with a smirk in Sherlock's direction.

"Have you ever used your powers since we met?" asked Sherlock. "Besides keeping the rain off."

"A couple times," John admitted. "A few weeks ago when Turner was about to shoot you, and the gun wouldn't go off; when I was tied up in the tramway with the crossbow pointed at Sarah, I used my telekinesis to untie the knots; and, uh…" He trailed off, smiling a little.

"What?" asked Sherlock.

John looked back at him. "I've shut the electricity off more than once to get you to go to sleep."

Sherlock narrowed his eyes. "Traitor."

John chuckled.

"What about in the army?" asked Sherlock.

"Well, any unit I was assigned to came to refer to me as a good luck charm," John replied. "When we came under fire, the enemy seemed to have a hard time hitting any of us. Any bombs we came across happened to be duds. Little things like that."

"What all can you do?" Sherlock asked.

"Well, I can manipulate water—move it, freeze it, heat it up—I can basically control the weather, I can turn invisible, and I have telekinesis," said John. "But I can also harden water to a glass-like consistency and generate and control electricity and fire."

"You can do all of that…and yet, you're brought down by one drop of water," said Sherlock.

"Seems unfair, doesn't it?" muttered John. After a moment, he realized Sherlock was staring at his legs. "You want to see the tail, don't you?"

"Could I?" asked Sherlock instantly, looking up at John.

"You're not going to peel scales off to study, are you?" asked John.

Sherlock frowned in a very offended manner. "I would never."

"Yeah, you would," muttered John as he got to his feet and moved to the cockpit, opening the door.

The pilot turned his head towards him.

"Could you stay in here for the next ten minutes or so?" John asked. "We're discussing a confidential case."

The pilot nodded. "Of course."

"Ta," said John.

He then moved to the back room of the plane and repeated the process with Mycroft's assistant. He returned to his seat and grabbed the glass of water from the cup holder by the window.

John looked down at Sherlock. "Mycroft doesn't have this thing bugged, does he?"

"Little late to be asking that," muttered Sherlock. "But, no, he doesn't."

"Right," said John, dipping a couple of fingers into the glass. He set it back in its holder and moved to the middle of the aisle. "Might want to watch out."

Sherlock—without taking his eyes off of John—turned his chair so that it was facing forward once again.

Another four seconds went by before the transformation began. John's body was suddenly enveloped by a column of water; or at least that's how it appeared. In reality, his body had essentially **become** water. As the water turned back to solid human being, John lost his balance and fell onto his back. Sherlock leaned over his armrest to get a look at John's transformed body. In place of legs, John now had a long merman tail, which had shiny white scales down the front with deep blue ones on the sides and around the fin.

Sherlock frowned at John's bare left shoulder as he propped himself up on his elbows on the floor. "Your scar is gone."

John glanced momentarily at his shoulder. "All part of the transformation."

"Hmm," mumbled Sherlock. He gestured at the tail. "May I?"

John waved a resigned hand. "Why not?"

Sherlock knelt in front of his chair, reaching out to feel the tail for himself. John's tail instinctively flinched at the touch, and Sherlock's eyes lit up.

"Incredible…" muttered Sherlock. "It's like one big muscle, whereas the human leg is made up of many." He looked up at John. "And whatever clothes you're wearing just…vanish?"

"Until I change back, yeah," John replied.

"Have you ever tried to take this away?" asked Sherlock. "Reverse the whole thing?"

"There were a couple times at med school when I went back to the cave on a full moon. I stayed in the pool all night while it came alive, but it didn't work. So, I decided to live with it." John then glanced towards the cockpit door he was facing. "I think we're cutting it close."

"Oh, of course," said Sherlock, taking his hand back.

John raised his hand over his tail and slowly closed it in a fist as steam began to rise from the tail. Another minute or so, and John—in his human form—was back. He stood and sat back down in his chair.

Sherlock sat back down as well. "Is Harry the only one who knows?"

John shook his head. "A few fellow army doctors while I was an Afghanistan, only people I knew I could trust."

"Of course." Sherlock nodded. "Should you ever be injured and come in contact with water as they're treating you."

John pointed at Sherlock in acknowledgement. "Which happened. They kept me in a private tent under the pretense that I had contracted a contagious infection when I was shot. I don't know what I would have done without them."

"You wouldn't have made it back to London," said Sherlock. "And if you did, you wouldn't have ever made it out of a military research facility."

"Yeah…" said John, staring at the floor and lost in the memories of that horrible week. "That's a scary thought…"

Sherlock stared at him a moment longer. "At least you don't have to come up with those ridiculous excuses for not doing the dishes anymore."

John looked up at him with an indignant huff. "Ridiculous?!"

"'I just put moisturizer on,'" Sherlock recited. "'Mrs. Hudson enjoys helping us out too much.' When has Mrs. Hudson _not_ told us she's not a housekeeper?"

"What was I supposed to say?" said John, half-laughing and half-protesting.

"Nothing," said Sherlock. "I never say anything about not doing the dishes."

John laughed. "We are so lucky we have Mrs. Hudson."

"My thoughts exactly," said Sherlock, turning towards him. "So, can you breathe underwater?"

John shook his head now that Sherlock had gotten right back around to being curious about his condition again. What had he gotten himself into?

* * *

 **So, this is going to be a series of oneshots, all part of the same story. They don't have to be in chronological order. If anyone has ideas of what they want to see happen with a merman-John flip to it or just extra ideas for this story, let me know! (I already have The Pool and Sherlock's fake death planned, so don't worry about those)**


End file.
